Laboratory and clinical findings suggest that states of low cerebral oxygenation are associated with cognitive decline and dementia in older adults. Sleep is a period of increased risk for hypoxic events; and low respiratory periodicity may be an important indicator for identifying individuals with low cerebral oxygenation during sleep. Our previous studies suggest that older adults with low respiratory periodicity have more fragmented sleep, more low amplitude delta sleep and greater declines in cerebral oxygenation during sleep. The aims of this longitudinal observational study are (1) to examine the relationship between measures of respiratory periodicity, sleep structure and cerebral oxygenation during sleep; (2) to examine whether respiratory periodicity or cerebral oxygenation during sleep, alone or in combination, predict cognitive decline over 24 months; and (3) to explore how changes in respiratory periodicity and cerebral oxygenation change over 24 months. One hundred and six community dwelling older adults, age 70+ years, will be evaluated at baseline and at 12-and 24-months. Respiratory periodicity will be measured from the respiratory inductance plethysmograph waveform and include the average 5- minute measures of the variability in the duration (sdIBI), frequency (sdIBF) and amplitude (sdAMP) of breathing cycles. Standard polysomnographic methods will be used to measure sleep structure, which include the proportion of sleep states, as well as the mean interval between sleep spindles, sleep-wake transitions, arousals, and K-complexes. Dual (right- left) cerebral oximetry will be used to measure maximum change in the average 5-minute right and left cerebral oxyhemoglobin saturation from resting awake baseline. Standardized tests of secondary memory, working memory, and attention will be measured at the same time as the sleep studies in order to describe changes in cognitive function and to examine whether respiratory periodicity or cerebral oxygenation alone or in combination predict cognitive decline over 24 months. In addition, we will explore how respiratory periodicity and cerebral oxygenation change and interact over time. Data on sleep quality, cerebrovascular risk factors, physical activity and depressive symptoms will be measured every 6 months because these measures can change rapidly and may affect the key outcome measures. In addition to serving as a comparison group, data from subjects who do not exhibit cognitive decline will be used in exploratory analyses to determine gender and race-associated differences in respiratory periodicity and cerebral oxygenation. Since little is known about respiratory periodicity or cerebral oxygenation using cerebral oximetry in older adults, this prospective study will provide important data for determining how these variables vary and may be used to predict cognitive decline in a carefully screened sample of older adults.